Showing papers on "Palladium published in 2005"
28 Jan 2005
1,119 citations
TL;DR: Acetoxylation of ethylene to vinyl acetate was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst, demonstrating that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers.
Abstract: Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single-crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers. The role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product, while inhibiting the formation of undesirable reaction by-products.
901 citations
TL;DR: The enhanced reactivity observed with these new catalytic systems has been attributed to the formation of unsaturated and reactive [PdL] species which can readily undergo oxidative addition reactions with ArX to yield [Pd(Ar)X(L)].
Abstract: Palladium-mediated cross-coupling reactions are attractive organometallic transformations for the generation of C--C, C--N, C--O, and C--S bonds. Despite being widely employed in small-scale syntheses, cross-coupling reactions have not found important industrial applications because until recently, only reactive aryl bromides and iodides could be used as substrates. These substrates are generally more expensive and less widely available than their chloride counterparts. Over the past few years, new catalytic systems with the ability to activate unreactive and sterically hindered aryl chlorides have been developed. The new catalysts are based on palladium complexes that contain electron-rich and bulky phosphine or carbene ligands. The enhanced reactivity observed with these new systems has been attributed to the formation of unsaturated and reactive [PdL] species which can readily undergo oxidative addition reactions with ArX to yield [Pd(Ar)X(L)].
657 citations
TL;DR: The rate-limiting step is the electrophilic attack of the palladium on an ortho arene C-H bond to form an agostic complex rather than a Wheland intermediate, and the cyclometalated product is formed by intramolecular deprotonation by acetate via a six-membered transition state.
Abstract: Various mechanisms for the cyclometalation of dimethylbenzylamine by palladium acetate have been studied by DFT calculations. Contrary to previous suggestions, the rate-limiting step is the electrophilic attack of the palladium on an ortho arene C−H bond to form an agostic complex rather than a Wheland intermediate. The cyclometalated product is then formed by intramolecular deprotonation by acetate via a six-membered transition state; this step has almost no activation barrier.
622 citations
551 citations
TL;DR: This work demonstrates the selective growth of patterned thin films of MOF-5 and the subsequent loading with a palladium organic compound, which acts as a precursor for the deposition of palladium nanoclusters inside the cavities of the MOf-5 material.
Abstract: Metal-Organic open Framework coordination polymers (MOFs) are a fascinating class of materials. We demonstrate the selective growth of patterned thin films of MOF-5 and the subsequent loading with a palladium organic compound, which acts as a precursor for the deposition of palladium nanoclusters inside the cavities of the MOF-5 material.
517 citations
505 citations
TL;DR: The functionalization of SBA-15 with mercaptopropyl trimethoxysilane leads to a material capable of absorbing Pd from organic and aqueous solutions, illustrating the importance of the thiol ligand to retain Pd on the surface.
Abstract: The functionalization of SBA-15 with mercaptopropyl trimethoxysilane leads to a material capable of absorbing Pd from organic and aqueous solutions. The resulting Pd-loaded material acts as a catalyst for the Suzuki−Miyaura and Mizoroki−Heck coupling reactions. Leaching studies show that the filtrate contains as little as 3 ppb Pd after reaction. Aminopropylated silica is an effective scavenger, and catalyst for the Mizoroki−Heck reaction, but leaching is significant (35 ppm), illustrating the importance of the thiol ligand to retain Pd on the surface. Heterogeneity tests such as hot filtration experiments and three-phase tests show that the reaction is occurring predominantly via surface-bound Pd.
500 citations
424 citations
418 citations
TL;DR: There are notable differences in reactivity between arylpalladium(II) intermediates generated by decarboxylative palladation and those produced in conventional Heck reactions, and it is found that more electron-rich alkenes react preferentially with an aryl group and trifluoroacetate intermediate formed by decarate palladium-based reactions, whereas an opposite trend is found inventional Heck reactions.
Abstract: Mechanistic studies of a palladium-mediated decarboxylative olefination of arene carboxylic acids are presented, providing spectroscopic and, in two instances, crystallographic evidence for intermediates in a proposed stepwise process. Sequentially, the proposed pathway involves carboxyl exchange between palladium(II) bis(trifluoroacetate) and an arene carboxylic acid substrate, rate-determining decarboxylation to form an arylpalladium(II) trifluoroacetate intermediate (containing two trans-disposed S-bound dimethyl sulfoxide ligands in a crystallographically characterized form), then olefin insertion and β-hydride elimination. Because of the unique mode of generation of the arylpalladium(II) trifluoroacetate intermediate, a species believed to be substantially electron-deficient relative to phosphine-containing arylpalladium(II) complexes previously studied, it has been possible to gain new insights into those steps that are common to the Heck reaction, namely, olefin insertion and β-hydride elimination....
TL;DR: Results strongly suggest that the Pd(0) nanoparticles serve as a reservoir of "homogeneous" catalytic active species.
Abstract: Pd(0) nanoparticles with ∼2 nm diameter, immobilized in 1-n-butyl-3-methylimidazolium hexafluorophosphate ionic liquid, are efficient catalyst precursors for coupling of aryl halides with n-butylacrylate. In situ TEM analysis of the ionic liquid catalytic solution after the catalytic reaction shows the formation of larger nanoparticles (∼6 nm). The palladium content in the organic phase during the arylation reaction was checked by ICP-AS and shows significant metal leaching (up 34%) from the ionic phase to the organic phase at low substrate conversions and drops to 5−8% leaching at higher conversions. These results strongly suggest that the Pd(0) nanoparticles serve as a reservoir of “homogeneous” catalytic active species.
TL;DR: It appears that this is a stereoselective trans alkene difunctionalization and thus a useful alternative to related cis-selective, metal-catalyzed alkene aminohydroxylation processes.
Abstract: A mild, palladium(II)-catalyzed ring-forming aminoacetoxylation of alkenes is described. Treatment of a range of nitrogen nucleophiles with catalytic palladium(II) in the presence of PhI(OAc)2 as oxidant resulted in alkene aminoacetoxylation, affording a variety of nitrogen-containing heterocycles. Our studies indicate the possibility for high levels of reaction regio- and stereocontrol. It appears that this is a stereoselective trans alkene difunctionalization and thus a useful alternative to related cis-selective, metal-catalyzed alkene aminohydroxylation processes.
TL;DR: Investigations on new catalyst systems and their application in organic synthesis are summarized.
TL;DR: The reaction is truly catalytic in metal catalyst and represents the proof of principle for this elusive type of alkene oxidation.
Abstract: Intramolecular diamination reactions are described which yield cyclic ureas as direct products of an oxidative alkene transformation in the presence of palladium acetate and iodosobenzene diacetate as terminal oxidant. The reaction is truly catalytic in metal catalyst and represents the proof of principle for this elusive type of alkene oxidation.
TL;DR: This communication describes the synthesis of a family of unusually stable palladium(IV) complexes containing two chelating 2-phenylpyridine ligands and two benzoates, and an unusual mechanism involving pre-equilibrium dissociation of the N-arm of the phenylpyrsidine ligand is proposed.
Abstract: This communication describes the synthesis of a family of unusually stable palladium(IV) complexes containing two chelating 2-phenylpyridine ligands and two benzoates. These complexes undergo clean C−O bond-forming reductive elimination upon heating, and the mechanism of this catalytically relevant process has been studied in detail. Solvent effects, crossover experiments, Eyring plots (which show ΔS⧧ of −1.4 ± 1.9 and 4.2 ± 1.4 in CDCl3 and DMSO, respectively), and Hammett analysis (which shows ρ = −1.36 ± 0.04 upon substitution of the para-benzoate substituent) all suggest that reductive elimination does not proceed via initial dissociation of a benzoate ligand. Instead, an unusual mechanism involving pre-equilibrium dissociation of the N-arm of the phenylpyridine ligand is proposed.
TL;DR: It is believed that palladium contaminants down to a level of 50 ppb found in commercially available sodium carbonate are responsible for the generation of the biaryl rather than, as previously suggested, an alternative non-palladium-mediated pathway.
Abstract: We present here a reassessment of our transition-metal free Suzuki-type coupling protocol. We believe that, although the reaction can be run without the need for addition of a metal catalyst, palladium contaminants down to a level of 50 ppb found in commercially available sodium carbonate are responsible for the generation of the biaryl rather than, as previously suggested, an alternative non-palladium-mediated pathway. We present a revised methodology for Suzuki couplings using ultralow palladium concentrations for use with aryl and vinyl boronic acids and discuss the effects of the purity of the boronic acid on the reaction.
TL;DR: This work has shown that CNT-supported monometallic and bimetallic nanoparticles synthesized in a water-in-hexane microemulsion may have a wide range of catalytic applications for chemical syntheses.
Abstract: Palladium, rhodium, and bimetallic Pd/Rh nanoparticles synthesized in a water-in-hexane microemulsion can be deposited directly on surfaces of functionalized multiwalled carbon nanotubes with high yields. The CNT-supported Pd nanoparticles are active catalysts for hydrogenation of olefins, for carbon-carbon bond formation, and for carbon-oxygen bond cleavage reactions. The CNT-supported Rh nanoparticles are active catalysts for hydrogenation of arenes, and the CNT-supported bimetallic Pd/Rh nanoparticles show an unusually high catalytic activity for hydrogenation of anthracene. This simple and novel synthetic technique for making CNT-supported monometallic and bimetallic nanoparticles may have a wide range of catalytic applications for chemical syntheses.
TL;DR: Palladium hydroxide on carbon (Pearlman's catalyst) effectively catalyzes direct arylation reactions of aryl iodides and bromides, providing excellent arieslation-to-hydrodehalogenation ratios (>30:1) with broad scope for both intra- and intermolecular arylations.
Abstract: [reactions: see text] Palladium hydroxide on carbon (Pearlman's catalyst) effectively catalyzes direct arylation reactions of aryl iodides and bromides, providing excellent arylation-to-hydrodehalogenation ratios (>30:1) with broad scope for both intra- and intermolecular arylation processes. Studies aimed at determining the nature of the active catalyst indicate that an active homogeneous palladium species is produced under the reaction conditions.
TL;DR: It is anticipated that refinements linking the vibrational spectrum of a probe molecule with surface structure and accessible adsorption sites for such a versatile catalytic substrate provide a platform against which structure/reactivity relationships can be usefully developed.
Abstract: Five alumina-supported palladium catalysts have been prepared from a range of precursor compounds [palladium(II) nitrate, palladium(II) chloride, palladium(II) acetylacetonate, and tetraamminepalladium(II) tetraazidopalladate(II)] and at different metal loadings (1–7.3wt%). Collectively, this series of catalysts provides a range of metal particle sizes (1.2–8.5nm) that emphasize different morphological aspects of the palladium crystallites. The infrared spectra of chemisorbed CO applied under pulse-flow conditions reveal distinct groupings between metal crystallites dominated by low index planes and those that feature predominantly corner/edge atoms. Temperature-programmed infrared spectroscopy establishes that the linear CO band can be resolved into contributions from corner atoms and a combination of (111)∕(111) and (111)∕(100) particle edges. Propene hydrogenation has been used as a preliminary assessment of catalytic performance for the 1wt% loaded catalysts, with the relative inactivity of the cataly...
TL;DR: The palladium-catalyzed vinylic substitution reaction has not yet received much attention from organic chemists, but its broad scope and simplicity demonstrate that it is a useful method for the synthesis of a variety of olefinic compounds as discussed by the authors.
Abstract: The palladium-catalyzed vinylation of organic halides provides a very convenient method for forming carboncarbon bonds at unsubstituted vinylic positions. Generally the reaction does not require anhydrous or anaerobic conditions although it is advisable to limit access of oxygen when arylphosphines are used as a component of the catalyst. The transformation is valuable because it cannot be carried out in a single step by any other known method (except in certain Meerwein reactions).
The organic halide employed is limited to aryl, heterocyclic, benzyl, or vinyl types, with bromides and iodides seen most often. Halides with an easily eliminated beta-hydrogen atom (i.e., alkyl derivatives) cannot be used since they form only olefins by elimination under the normal reaction conditions. The base needed may be a secondary or tertiary amine, sodium or potassium acetate, carbonate, or bicarbonate.
When nucleophilic secondary amines are used as coreactants with most vinylic halides, a variation occurs that often produces tertiary allylic amines as major products. The catalyst is commonly palladium acetate, although palladium chloride or preformed triarylphosphine palladium complexes, as well as palladium on charcoal, have been used. A reactant, product, or solvent may serve as the ligand in reactions involving organic iodides, but generally a triarylphosphine or a secondary amine is required when organic bromides are used. The reaction, which occurs between ca. 50° and 160° proceeds homogeneously. Solvents such as acetonitrile, dimethylformamide, hexamethylphosphoramide, N-methylpyrrolidinone, and methanol have been used, but are often not necessary. The procedure is applicable to a very wide range of reactants and yields are generally good to excellent.
Several variations of the reaction are known in which the organic halide is replaced by other reagents such as organometallics, diazonium salts, or aromatic hydrocarbons. These reactions are not discussed in detail, but are only briefly compared with the halide reaction. Other related reactions such as the palladium-catalyzed replacement of allylic substituents with carbanionic reagents, the palladium-promoted nucleophilic substitutions at olefinic carbons, and the numerous palladium-catalyzed coupling reactions of halides and organometallics are also beyond the scope of this review. The palladium-catalyzed vinylic substitution reaction has not yet received much attention from organic chemists, but its broad scope and simplicity demonstrate that it is a useful method for the synthesis of a variety of olefinic compounds.
Keywords:
palladium catalyst;
vinylation;
organic halides;
scope;
limitations;
experimental procedures;
vinylic substitution;
ethylene;
acrylic acid;
butenol;
methyl acrylate;
styrene;
pentadiene;
acrolein dimethyl acetal;
olefins
TL;DR: Palladium nanoparticles were deposited inside multi-walled carbon nanotubes (MWNTs) via a simple impregnation using an aqueous solution containing a palladium salt as mentioned in this paper.
Abstract: Palladium nanoparticles (4–6 nm) were deposited inside multi-walled carbon nanotubes (MWNTs) via a simple impregnation using an aqueous solution containing a palladium salt. The low surface tension of the solvent allows a complete filling of the tube, leading, after thermal treatments, to the formation of small and homogeneous palladium particles decorating the inner cavity of the support. The impregnation method was extremely efficient as no palladium particle located on the outer surface of the tubes was observed. The catalyst was tested for the selective hydrogenation of cinnamaldehyde which contains both a C C and a C O bond. The nanotubes based catalyst exhibits along with a high catalytic activity an extremely high selectivity towards the C C bond hydrogenation when compared to a commercial catalyst supported on a high surface area activated charcoal. A peculiar metal-support interaction and the absence of micropores and of oxygenated surface groups on the carbon nanotubes support are proposed to explain these results.
TL;DR: In this article, the authors explored the behavior of palladium-based anode catalyst for direct methanol and Direct Formic acid fuel cells (DFAFCs), and showed that the performance loss of DFAFC can be fully recovered by applying a positive potential at the fuel cell anode after short-term life test.
TL;DR: In this article, the authors used palladium well-dispersed on titanium nanotubes, in relation to methanol oxidation processes in the direct oxidation methanoline fuel cell.
TL;DR: In this paper, a siloxane self-assembled monolayer placed between a substrate and an evaporated ultrathin Pd film promotes the formation of small Pd nanoclusters and reduces the stiction between the palladium and the substrate.
Abstract: Resistive-type palladium structures for hydrogen sensing remains as a research focus for their simplicity in device construction. We demonstrate that a siloxane self-assembled monolayer placed between a substrate and an evaporated ultrathin Pd film promotes the formation of small Pd nanoclusters and reduces the stiction between the palladium and the substrate. The resulting Pd nanocluster film can detect 2%H2 with a rapid response time of ∼70ms and is sensitive to 25 ppm hydrogen, detectable by a 2% increase in conductance due to the hydrogen-induced palladium lattice expansion.
TL;DR: Poly(1-methylimidazoliummethyl styrene)-surface grafted-poly(styrene) resin was prepared for the first time as a polymer-supported N-heterocyclic carbene (NHC) precursor for palladium complex by suspension polymerization.
Abstract: Poly(1-methylimidazoliummethyl styrene)-surface grafted-poly(styrene) resin was prepared for the first time as a polymer-supported N-heterocyclic carbene (NHC) precursor for palladium complex by suspension polymerization. To prepare this polymer-supported NHC precursor, 1-methyl-3-(4-vinylbenzyl)imidazolium hexafluorophosphate, [MVBIM][PF6-], was synthesized as a monomer and copolymerized with styrene and DVB in water. This polymer-supported NHC precursor with imidazolium as a ligand, which exists solely on the surface of the resin, was well characterized by FE-SEM, CLSM, and IR spectroscopy. The precursor containing imidazolium readily formed a stable complex with Pd(OAc)2, and this polymer-supported N-heterocyclic carbene-palladium complex exhibited excellent catalytic activity for Suzuki cross-coupling reaction in an aqueous medium. The catalyst was recovered quantitatively from the reaction mixture by simple filtration and was able to be reused for a number of recycles with consistent activity in all of the coupling reactions.
TL;DR: In this paper, the hydrogen storage capacities of palladium-and vanadium-doped carbon nanotubes (CNTs) at room temperature were investigated using the Sieverts apparatus.
TL;DR: In this article, the reduction of Pd 2+ to nano Pd was sensitive to the chain length of the polyethyleneglycol (PEG) and the larger chain length PEG such as PEG 2000 exhibited higher reduction reactivity.
Abstract: Palladium nanoparticles with narrow size distribution were prepared by applying poly(ethyleneglycol) (PEG) and Pd(OAc) 2 in the absence of other chemical agents. PEG appeared to act as both reducing agent and stabilizer. The results of XRD show that the concentration of precursor played the key role to the development of palladium nanoparticles. The reduction of Pd 2+ to nano Pd was sensitive to the chain length of the PEG, and the larger chain length PEG such as PEG 2000 exhibited higher reduction reactivity. The as-prepared palladium nanoparticles were found to be a highly stable and reusable catalyst for Heck reaction.